The invention relates to a method of producing a composite ultrasonic transducer of the type which includes transducer elements that are dispersed in plastic, radiate essentially in the longitudinal direction, and comprise piezoelectric ceramic.
A composite ultrasonic transducer of this type is constructed from numerous small, piezoelectrically-active transducer elements. The dimensions of the transducer elements are conceptualized such that the elements radiate essentially in the longitudinal direction. The transducer elements of a composite ultrasonic transducer are dispersed in a plastic matrix such that their longitudinal directions are parallel. The length of the individual transducer elements determines the thickness of the composite ultrasonic transducer and thus the working-frequency range.
In a method of producing such a composite ultrasonic transducer as disclosed in EP 0 462 311 B1, first a plastic mold is created that contains negative structures corresponding to a predetermined arrangement of the transducer elements, with the mold projecting beyond the negative structures. The mold is filled with a ceramic slip to cover the negative structures, and the slip is then dried and fired. During firing, the plastic mold is burned up without solid residue, and the fixed transducer-element arrangement now appears on a ceramic base. A polymer is poured into the hollow spaces formed during firing by the burning of the plastic mold. The polymer fixes the position of the transducer elements and provides the mechanical stability of the composite ultrasonic transducer while meeting the acoustical requirements. Finally, the ceramic base connecting the transducer elements is removed, and electrodes are positioned on the end faces of the transducer elements.
In this method, for clean and loss-free unmolding of the plastic mold with the negative structures of the transducer arrangement, it is necessary to longitudinally taper the cross section of the negative structures that preset the spaces between the individual transducer elements. Consequently, only ultrasonic transducers having frustoconical or truncated-pyramidal transducer elements can be produced with this method. In addition, only limited ratios between the transducer-element geometry and the distances between the transducer elements can be realized, with the ceramic proportion being relatively small, and therefore only permitting a limited acoustical capability.
In another known method of producing composite transducers, longitudinal and transverse slots are sawed, with a highly-precise ceramic saw, preferably a circular band saw, into a ceramic block, a so-called blank, that is produced in accordance with a suitable method. The sawing cut is only deep enough that a continuous, lower ceramic base remains. The used blank is cast with a polymer, preferably polyurethane. Following the casting, the ceramic base is sawed off. The depth of the sawing cuts made in the blank is determined by the desired working frequency (resonance frequency) of the transducer.
The disadvantage of this method lies in the lengthy processing times for the sawing. Furthermore, the reject rate is very high, because some of the sawed columns break very easily due to the brittleness of the ceramic material, rendering the entire blank unusable.
It is the object of the invention to provide a method of producing a composite ultrasonic transducer that is not subject to the above limitations and, because of shorter processing times and an extremely-low reject rate, is economical.